Porous burner as well as a method for operating a porous burner

ABSTRACT

The invention relates to a porous burner with a housing, which has an inlet for a fuel-air mixture and an outlet for the exhaust gas mixture generated in the burner, where in flow-direction of the fuel-air mixture the housing contains an ignition space with an ignition device and adjacent to this space a porous burner medium. On the inlet side the ignition space is provided with a stabilizing element, which reduces the inlet cross-section and directs the flow of the fuel-air mixture essentially perpendicular to the inlet cross-section of the porous medium. The porous burner is provided with a device for controlling the mass flow of the fuel-air mixture, which serves to shift the combustion zone from the ignition space into the porous medium.

BACKGROUND OF THE INVENTION

The invention relates to a porous burner with a housing, which is provided with an inlet for a fuel-air mixture and an outlet for the exhaust gas mixture generated in the burner, and contains in flow direction of the fuel-air mixture an ignition space with an ignition device and adjacent thereto a porous body. The invention further relates to a method of operating a porous burner of this kind.

In contrast to conventional combustion using a free flame, the combustion reactions of combustion in porous inert media do not take place in a free flow of gas but within a coherent structure of cavities of an inert porous body. Stabilization in the porous matrix is made possible by its significantly better heat transport properties as compared to the pure gas phase. Gas-phase and solid state matrix are approximately in thermal equilibrium and thus no free flames will occur.

The operation of conventional porous burners generally shows the following operational phases.

-   -   Start-up: during the start-up phase the burner is brought to         operating temperature by the hot exhaust gases of free         combustion taking place in front of the porous body or medium.         To generate the start-up flames pressurized oil atomizer nozzles         may be employed.     -   Intermediate phase: the start-up phase is terminated by a short         interruption of fuel feed.     -   Steady-state operation: with renewed fuel feed the fuel-air         mixture is brought to combustion by self-ignition in the hot         porous medium. The combustion zone is thus stabilized in the         porous medium.

DESCRIPTION OF PRIOR ART

DE 102 28 411 C1 describes a porous burner with reduced start-up emissions, in which after start-up no intermediary phase is required during which fuel feed would have to be interrupted. The burner has a housing with an inlet for the fuel-air mixture and an outlet for the hot smoke gases, with finely porous material being provided in a zone on the inlet side and coarsely porous material being provided in a zone on the outlet side. The burner is furthermore provided with a shifting device by means of which a free intermediary space can be created between the finely porous and the coarsely porous zone during the start-up process. In the start-up phase the fuel-air mixture is fed through the finely porous zone into the free intermediary space where it is burnt. The smoke gases generated by combustion in the free intermediary space will heat the coarsely porous region to operating temperature, which region does not take part in the combustion process during start-up. After the start-up phase, when optimum operating temperatures have been reached in the second zone, the coarsely porous material is moved back towards the finely porous material of the first zone. The presence of movable parts in the burner and their actuating means is of disadvantage, since they will lead to increased maintenance expense in addition to greater manufacturing and operating costs of the porous burner.

In this context there is known from DE 197 29 718 A1 a combustor body for a burner for gaseous fuels, where at the inlet of the burner in flow direction of the process gases a first zone of porous material is provided in the form of “spaghetti-ceramics”. This is followed by a free ignition chamber with an ignition device, and behind the ignition chamber there is located an element consisting of a plurality of corrugated sheet metal platelets placed side by side. The faces of the platelets are parallel to the flow direction of the gas-air mixture. During operation of the burner the gas-air mixture ignites in the open ignition chamber, and the flame front propagates downstream, heating the element consisting of corrugated platelets and stabilizing itself in this material.

Furthermore DE 43 22 109 A1 describes a burner in which the combustion chamber is filled with porous material whose porosity changes along the length of the combustion chamber in such a way that porous cavity size increases in flow direction of the process gases, resulting in optimum parameters for porous cavity size and thus flame development at a boundary surface or in a certain zone of the porous material, which will permit a flame to develop. Flame stabilization thus occurs at the transition from a finely porous to a coarsely porous medium.

Finally, DE 199 60 093 A1 describes a gas burner and a method for the flameless combustion of a gas-air mixture. The burner consists of a hollow cylindrical perforated plate and a porous body placed concentrically and at a certain distance above the plate. The gas is fed into the cavity between the hollow cylindrical perforated plate and the porous medium, and the oxidizing air is directly fed into the hollow cylindrical perforated plate. The reaction zone of this gas burner is permanently located in the cavity.

SUMMARY OF THE INVENTION

It is the object of the present invention to improve a porous burner of the initially described kind and a method of operating this burner in such a way that the burner has no movable parts and that noxious emissions are minimized in the start-up phase as well as during steady-state operation. Furthermore, this burner design should be applicable for gaseous fuels as well as oil, and the transition from start-up to steady-state operation should be controllable without problems.

According to the invention this object is achieved by providing that the ignition space has a stabilizing element at the inlet side, which reduces the inlet cross-section and directs the flow of the fuel-air mixture essentially at a right angle to the inlet cross-section of the porous medium, and that the porous burner is furnished with a device for controlling the mass-flow of the fuel-air mixture, which serves to shift the combustion zone from the ignition space into the porous medium.

A method according to the invention for operating such a burner, which is provided with an ignition space with an ignition device in front of a porous medium in flow direction of the fuel-air mixture, is characterized by the following steps:

-   -   Essentially perpendicular direction and acceleration of the         fuel-air mixture by means of a perforated plate at the inlet of         the ignition space;     -   Ignition of the fuel-air mixture in the ignition space and         stabilization of a flame front in a combustion zone on the side         of the perforated plate facing the porous medium;     -   Increasing the mass-flow of the fuel-air mixture and shifting         the combustion zone into the porous medium.

In the case of an oil-burner the fuel-air mixture is prepared in a preferably heatable mixing chamber preceding the perforated plate.

According to the invention the stabilizing element consists of a perforated plate made of ceramics, with a flow cross-section amounting to 10% to 30%, and preferably 15% to 20%, of the free flow cross-section of the porous burner.

The working principle of the porous burner according to the invention will now be described for the case of an oil burner.

Pretreatment of the combustion air and the fuel, i.e., heating of both components to target temperature and evaporation of the oil, takes place in a mixing chamber preceding the porous burner itself (see AT 408 904 B, for example). The generated turbulent oil-vapor air mixture then flows through the perforated ceramic plate, which has a significantly smaller flow cross-section than the porous burner and will thus accelerate the fuel-air mixture and give it an axis-parallel flow direction in the case of a cylindrical burner.

During start-up the burner is operated at low power and with a low excess air ratio (e.g., λ=1.1), resulting in low mass-flow of the fuel-air mixture and in a low exit velocity from the perforated ceramic plate. With increasing process temperature the flame velocities also increase and the combustion zone shifts upstream from the ignition site towards the perforated plate. A flame front consisting of a plurality of single flames is established, which is stabilized by the perforated plate. The hot smoke gases generated by combustion will flow through the porous medium and heat it. Subsequently, the excess air ratio λ is increased while power is kept constant, which causes the mass flow and thus the exit velocity of the fuel-air mixture to increase. The combustion zone now shifts downstream into the porous medium and is stabilized there. Steady-state operation of the burner has set in. The desired steady-state power output may now be set with a corresponding limiting excess air ratio. The limiting excess air ratio is given by the amount of combustion air necessary for shifting the combustion zone completely into the porous medium and stabilizing it there, keeping all noxious emissions below their prescribed threshold values.

As an example the operating sequence for an oil burner (for instance a burner for domestic fuel oil) with a power range from 2 to 16 KW will now be described.

-   -   Preheating of the mixing chamber: the mixing chamber is first         preheated by an electric heating element to a temperature of         350° C. After a temperature of 230° C. has been reached         combustion air is fed into the mixing chamber, which flushes         combustion residues from the porous medium. At a mixing chamber         temperature of 345° C. high-voltage spark ignition is activated.     -   Start-up phase (ignition and heating of the porous medium): at         350° C. the fuel-air mixture with an excess air ratio of λ=1.1         (at a power output of 5.5 KW) is fed into the ignition space and         ignited by the high-voltage spark ignition device. The         combustion zone will then move upstream with increasing flame         temperature towards the perforated ceramic plate and develop a         flame front on the side of the perforated plate facing the         porous medium. The porous medium is heated by the hot smoke         gases.     -   Shifting of the combustion zone: the excess air ratio is now         increased to λ>1.1. Due to the increased mass flow of the         mixture the exit velocity also increases. The flame front         stabilized by the perforated plate becomes unstable and the         combustion zone finally shifts into the porous medium.     -   Steady-state operation: the burner is now adjusted to deliver         the desired steady-state power output. The necessary excess air         ratios are to be chosen depending on the given power value.

In addition to the tasks of stabilizing, respectively destabilizing, the starting flame as described above, the perforated ceramic plate has the following functions:

-   -   Avoiding flame blowback;     -   Redirecting the originally circular flow of the fuel-air mixture         in such a way that it moves essentially perpendicular to the         inlet cross-section of the porous medium;     -   Protecting the mixing chamber against overheating due to         radiation from the porous medium.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained in more detail with reference to the enclosed drawings, wherein

FIG. 1 shows a porous burner according to the invention in a longitudinal section;

FIG. 2 a variant of the porous burner of FIG. 1 with a mixing chamber placed in front, in a sectional view;

FIG. 3 a diagram showing the relationship between the limiting excess-air ratio λ_(k) and the heating power of the porous burner; and

FIG. 4 a variant of the porous burner according to FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The porous burner shown in FIG. 1 (for instance, a gas burner) has a housing 2 with an inlet 3 for a fuel-air mixture 4 and an outlet 5 for the exhaust gas mixture 6 generated in the burner, where in flow direction of the process gases there is provided in the housing 2 an ignition space 7 with an ignition device 8 followed by a ceramic porous medium 9. The ignition space 7 is bounded on the inlet side by a stabilizing element 10, a perforated ceramic plate, which reduces the inlet cross-section and redirects the flow of the fuel-air mixture 4 in such a way that it is perpendicular to the inlet cross-section of the porous medium 9. In the example shown, which is a cylindrical porous burner, the fuel-air mixture is directed parallel to the axis 1′ of the porous burner 1. The porous burner 1 furthermore is provided with a device 11 for controlling the mass flow of the fuel-air mixture 4, for instance a pressure blower in front of the burner or a suction fan behind the burner. Preferably, control is effected by increasing the excess air ratio λ when the mass flow of the fuel-air mixture 4 is to be increased at the transition from the start-up phase to steady-state operation.

The housing 2 of the porous burner 1 essentially comprises a water-cooled combustion pipe 12, which is provided with a heat-insulating lining 13. The perforated ceramic plate 10 is held in a conical opening at the inlet 3 of the porous burner 1 by means of a conical clamping ring 14.

In the porous oil-burner shown in FIG. 2 a preferably heatable mixing chamber 15 for pretreating the combustion air and a liquid fuel is placed in flow direction of the process gases in front of the stabilizing element 10. The heater element 16 of the mixing chamber 15 is positioned in the area of the inlet opening 18, the mixing chamber itself being attached to the water-cooled combustion pipe 12 via an intermediate ring 17.

The material data and numerical values cited in the following pertain to a concrete embodiment and are not to be interpreted as restrictions for the present invention.

Regarding design and material, the porous medium 9, in which process temperatures between 1500° C. and 1800° C. can be attained, corresponds to a standard model of porous burner technology. For example, corrugated Al₂O₃ ceramics could be used. The porous medium could also consist of ZrO₂, SiO₂ or other high-temperature ceramics. The heat-insulating lining 13 has a thickness of 5 mm, for instance, and is made of a material with low heat conductivity (e.g., 0.85 W/mK at 745° C.), with a small expansion coefficient (e.g. 0.9*10-61/K) and a porosity of 20%. The stabilizing element 10 is a circular, perforated ceramic plate with an outer diameter of 67 mm, a thickness of 22 mm and a reduced free cross-section area of 689 mm², the individual bores 10′ of the plate having a diameter of 2.19 mm. Axial directing of the fuel-air mixture is achieved by means of the relatively long axial bores 10′, the diameter of the bores 10′ in the perforated plate 10 being such that in case of a flame blowback the flames are quenched. The diameter of the bores 10′ amounts to 80% to 15%, and preferably 100%, of the thickness of the perforated plate 10, for instance.

In the example shown the circular perforated ceramic plate 10 is bedded in a conical brass clamping ring and via the ring attached to the inlet 3 of the combustion pipe 12. The conical clamping ring 14 is pressed into the conical opening of the combustion pipe 12 by the intermediate ring 17 and is thereby held in place. It is also possible to manufacture a perforated plate with a conical rim which can be directly inserted into the conical opening of the combustion pipe 12. The whole attaching system of the perforated plate is kept almost free of heat-expansion due to the watercooling of the combustion pipe 12, and thus the stabilizing element 10 can withstand even highest temperature loads. At the same time the watercooling prevents overheating of the stabilizing element 10 and an undesirable, premature self-ignition of the fuel-air mixture.

The shifting of the combustion zone from the side of the perforated plate 10 facing the ignition space 7 into the porous medium 9 is caused by destabilization of the starting flames. This is achieved when the flow velocity exceeds the flame velocity at all points of the flame front. While sufficiently high total mass flow can be attained with low excess air ratios in the high power range of the burner, a higher proportion of combustion air in the mixture is required at low power, which is associated with decreased fuel mass flow. This defines a power-dependent limiting excess air ratio λ_(k) as shown in FIG. 3.

In the embodiment shown in FIG. 4 the perforated plate 10 is configured in two parts and is held by two clamping flanges 19, 19′ and clamping brackets 20. The front clamping flange 19 is also used to attach the mixing chamber 15. 

1. A porous burner comprising a housing, which housing has an inlet for a fuel-air mixture and an outlet for the exhaust gas mixture generated in the burner, an ignition space with an ignition device and, following said ignition space, a porous medium being provided in the housing in flow direction of the fuel-air mixture, wherein the ignition space contains a stabilizing element at its inlet side, which reduces the inlet cross-section and directs the flow of the fuel-air mixture essentially perpendicular to the inlet cross-section of the porous medium, and wherein the porous burner is provided with a device for controlling the mass flow of the fuel-air mixture.
 2. A porous burner according to claim 1, wherein in front of the stabilizing element in flow direction of the fuel-air mixture, there is located a mixing chamber for pretreatment of combustion air and liquid fuel
 3. A porous burner according to claim 2, wherein the mixing chamber is heatable.
 4. A porous burner according to claim 1, wherein the stabilizing element is a perforated ceramic plate.
 5. A porous burner according to claim 4, wherein the perforated ceramic plate has a flow cross-section which amounts to 10% to 30% of the free flow cross-section of the porous burner.
 6. A porous burner according to claim 4, wherein the perforated ceramic plate has a flow cross-section which amounts to 15% to 20% of the free flow cross-section of the porous burner.
 7. A porous burner according to claim 4, wherein the diameters of the bores in the perforated plate are dimensioned such that the flames are quenched in the case of flame blowback, with the diameters of the bores amounting to 8% to 15% of the thickness of the perforated plate.
 8. A porous burner according to claim 4, wherein the diameters of the bores in the perforated plate are dimensioned such that the flames are quenched in the case of flame blowback, with the diameters of the bores amounting to approximately 10%, of the thickness of the perforated plate.
 9. A porous burner according to claim 4, wherein the perforated ceramic plate is attached to the inlet of the porous burner by means of a conical clamping ring.
 10. Method for operating a porous burner, which has an ignition space with an ignition device in flow direction of the fuel-air mixture in front of a porous medium, comprising: a) directing and accelerating the flow of the fuel-air mixture essentially perpendicular to the inlet cross-section of the porous medium by means of a perforated plate at the inlet of the ignition space; b) ignition of the fuel-air mixture in the ignition space and stabilization of a flame front in a combustion zone on the side of the perforated plate facing the porous medium; c) increasing the mass-flow of the fuel-air mixture and shifting the combustion zone into the porous medium.
 11. Method according to claim 10, wherein in the case of liquid fuels the fuel-air mixture is prepared in a heatable mixing chamber located in front of the perforated plate.
 12. Method according to claim 10, wherein the air ratio λ is increased for the purpose of increasing mass flow of the fuel-air mixture at the transition from start-up phase to steady-state operation. 